Descaling metals and alloys with aqueous potassium hydroxide at relatively low temperature



United States Patent Office EJ121326 Patented Feb. 11, 1964 DESCALING PETALS AND ALLQYS WITH AQUE- GUE: PQTASSIUM HYBRflXlDE AT RELATIVELY LOW TED/IPEPATURE Jack M. Beigay, Breckenridge, Roy C. Bongartz, Pitt'shurgh, and Donald R. Zaremslsi, ilheswiclr, Pm, assignors to Titanium Metals Corporation of America, New York, N.Y., a corporation of Delaware No Drawing. Filed .luly 15, 1960, Ser. No. 42,964

8 Claims. ((1 134-2) This invention relates to the descaling of metals and alloys and particularly to a preliminary descaling treatment for metals and alloys that form tightly adhering, acid resistant complex oxide scales upon heat treatment. In the manufacture of metal shapes and metal objects it is necessary to frequently heat treat the metal between and during processing and fabricating operations. Such heat treatment is generally conducted in air or in an oxidizing atmosphere and results in a metal oxide film on the surface of the metal object commonly referred to as scale. Numerous means have been employed her tofore in attempting to remove such scale from metals prior to subsequent processing or fabricating steps, the most common and successful of which is an aqueous acid pickle. The exact pickling solution, temperature and time employed varies according to the type of metal or alloy being treated, the heat treatment to which such metal or alloy was subjected, etc. Some metals and alloys, particularly the reactive metals titanium, titanium alloys and steels with high chromium content such as stainless steels and superalloys, form oxide scales that are substantially acid resistant and are extremely ditlicult to pickle in aqueous acid solutions. To remove such a complex tightly adhering oxide scale such metals and alloys may be given a descaling treatment of pro-treatment which consists of immersing the metal or alloy in a molten caustic salt which either removes the oxide scale or so reaks it down as to render it susceptible to subsequent acid pickling. The molten caustic salt is generally composed of alkali metal hydroxides plus small additions of oxidizers, etc., and requires temperatures of from 809 F. to 1000 F. to render it molten and reactive for descaling or pro-descaling treatment. Such treatment in molten caustic salts frequently results in detrimental chemical or electro-chemical attack of metal surfaces and is sometimes unsuccessful in descaling or pre-descaling of scaled metals. For example, titanium alloy, Type Ti 140A (Ti2% Fe2% Cr2% Mo), exhibits a groove type of chemical attack when immersed in molten Virgo (commercial salt that consists essentially of sodium hydroxide). Titanium alloy metal products, such as Ti4-% Al-3% Mo-2% V all exhibit pit type of chemical attack when immersed in the commonly used commercial molten salts. The surfaces of chromium-manganese stainless steels are etched when immersed in molten Virgo salts. Oxide scale is not completely removed from titanium alloys, such as Ti2% Fe2% Cr2% Mo and Ti5% Al2 /2% Sn, when immersed in molten Virgo and subsequently acid pickled. Pit-ting of these metals generally occurs upon recycling to remove the unafiected scale. The commonly used molten caustic salt baths have very little efiect on the oxide scale of straight chromium stainless steels. Also, the high temperature required for such molten salt baths create undesirable heating efiects, particularly on stainless steels.

It has now been found that tightly adhering, acid resistant, complex metal oxides may be efiectively removed from metal objects by treatment in an aqueous potassium hydroxide solution at relatively low temperatures and without corrosive chemical attack of the metal surface.

In general this invention relates to the treatment of metals and alloys having metal oxide scales on their surfaces by immersing such metals and alloys in an aqueous bath that contains potassium hydroxide. Difficult to remove oxide scale is removed from metal by immersing the scaled metal in an aqueous solution that contains from 60% to KOH at a temperature of from 300 to 450 F. The treatment is effective in some cases as a descaling step and in other cases as a pre-treatment to descaling in an acid bath. Also, grind lines may be removed from metals and alloys by a like treatment. The treatment of the present invention is particularly applicable to metals and alloys that develop complex and acid resistant scales when heated in an oxidizing atmosphere, such as the reactive metals titanium and titanium alloys, and steels containing a chromium content in excess of 10%, such as stainless steels and superalloys.

It is therefore the object of the present invention to provide a method of chemically treating metals and alloys having complex and acid resistant oxide scale thereon to remove or facilitate the removal of such scale.

It is also an object of the present invention to provide a method to chemically remove tightly adhering and acid resistant oxide scales from metals or alloys without detrimental chemical or electrochemical attack of the metal or alloy surface.

It is also an object of the present invention to provide a method for removing grind lines from metals and alloys without detrimental chemical attack of the surface of the metals and alloys.

A further object of the present invention is to provide a low temperature descaling treatment or pre-treatment for metals and alloys that form tightly adhering acid resistant scales.

A still further object of the present invention is to pro vide a potassium hydroxide bath for the removal or altering of complex and acid resistant oxide scales on the surface of metals or alloys.

A more specific object of the present invention is to provide a method of descaling or a pre-treatment for descaling of scaled reactive metals such as titanium, titanium alloys, and chromium containing alloys such as stainless steels and superalloys.

Other objects and advantages can best be understood from the following description.

Although the aqueous potassium hydroxide metal treat ment solutions of the present invention may be crystalline or solid at room temperature in the more concentrated range, and are liquid upon heating to the operating temperature, they are not to be confused with the fused salt baths which also may be composed mainly of potassium hydroxide (KGH) but which are heated to a molten condition for metal treatment. Such baths are substantially anhydrous containing a minimum of Water. The descaling baths of the present invention contain at least 10%, by Weight, Water. The solubility of potassium hydroxide is such that combinations of KOH and 10% E 0 are not liquid at room temperature. When heated to about 350 F. the KOH, although comprising about 90%, of the mixture becomes soluble in the water present thus forming a liquid suitable for metal treatment. Concentrations of KOH greater than 90% are not preferred in that such high concentrations are relatively chemically inactive and effect a reduced descaling action. Concentrations of less than 60% KOH are not strong enough to eifectively descale or alter the characteristics of heavy, acid-resistant scales Within a reasonable time. The process of this invention therefore contemplates employment of aqueous baths containing from 60% to 90% by weight potassium hydroxide. Excellent results have been obtained with baths containing 0% commercial grade KOH with the balance water.

The presence of free or uncombined hydrogen is undeshable in either the conventional fused salt baths or the aqueous potassium hydroxide baths of the present invention. The origin of such hydrogen is not conclusively known but is thought to come from the dissociation of materials in the bath due to reactions of metal with the salt. The presence of such hydrogen frequently causes hydrogen penetration of the metal being treated which is undesirable from the standpoint of possible hydrogen embrittlement. At any rate the descaling bath of the present invention provides essentially no hydrogen contamination.

It has been noted that the hydrogen absorption rate of alloys containing aluminum and tin are adversely high after treatment in the baths of the present invention. For example, some or" the baths of the present invention have proved to be detrimental in some respects to Ti5% Al2 /2% Sn alloy due to the absorption of liberated hydrogen during the treatment of this metal. It has been found that appropriate additions of oxidizing agents to react with the hydrogen have eiiectively minimized the effect of hydrogen contamination.

The oxidizing addition employed may vary with the type of metal being treated, oxidizing material available, etc.; however, where such additions are found necessary, excellent results may be obtained by additions of nitrates. For example, in an 80% KOH, balance water bath, a 10%, by weight, addition of potassium nitrate effectively prevents the absorption of liberated hydrogen on Ti5% A1-2 /2% Sn alloys. The concentration of oxidizing agents is not critical. Additions of from 6% to 50%, by weight, of KNO have been found to be elfective in 80% KOH baths. However, practical considerations such as relative costs of materials make it impractical to add more than about 30% of KNO to the bath. Other nitrate additions, such as NaNO may be equally as eifective as KNO if added in equivalent quantities in respect to the N content.

The potassium hydroxide employed for the baths of the present invention is not required to be chemically pure but may contain impurities such as are common to commercially available salts of this type. For example, chemical analysis or" a bath containing 80%, by weight, commercially available potassium hydroxide as employed in the present invention showed that only about 69% active KOH was present, leaving a balance of about 11% impurities. However, the bath operated effectively in removing scale from titanium and stainless steel.

The preferred operating temperature is from about 300 F. to 450 F. A temperature of at least about 300 F. to 350 F. is required to provide solubility of KOH in the water present in compositions containing 60% to 90% potassium hydroxide. Also, the time involved when temperatures below 300 F. are employed becomes excessive and impractical in the commercial treatment of metals and alloys. Temperatures above 450 F. cause excessive reaction and probable detrimental chemical attack of the surface of the metals or alloys being treated.

The time such metals and alloys are being treated is not a critical factor in itself; however, practical considerations, such as the desire to continuously conduct metfl strip through a descaling or pro-descaling bath and the economic factor of maintaining the metals and alloys in such a solution, dictate a preferred time of from about to 25 minutes. Optimum results in removing oxide scale and in removing grind lines from titanium alloy materials have been secured from 5 to minutes in an 80%, by weight, potassium hydroxide solution plus water at a temperature of about 400 F.

The following are illustrative examples of the process Metal or Alloy Heat Treatment Ti-6%A1-4%V Ti5%Al-2%%Sn Ti2 %Fe2%Cr2%M0 Ti6% Al-4 %V Ti-75A (Commercial Titanium) Stainless Steel, Type 430 Do Stainless Steel, T e 302 D0 Stainless Steel, Tyo

D0 Stainless Steel, Stainless Steel, Type 3l0 Stainless Steel, Type 316 Do .l Stainless Steel, Type 347-. Stainless Steel, Type 350 Stainless Steel, Type 355 Stainless Steel, Type 142 Stainless Steel, Type 446.. Stainless Steel, Type 201 H0t)Ro1led 1,7oo r.-1,750

L 11 Rolled 1,900 F.-1,950

Box and Mutfie Annealed (l,300 F.1,l00 1 Solution Treated (1,720 F.). Lab Annealed (1,400 P). Lab Annealed (1,S00 Mill Annealed (1,950 Lab Annealed (1,800" 3.

Mill Annealed (1,: Lab Annealed (1,80 Mill Annealed (l,950

D0. Lab Annealed (1,800 I Mill Annealed (1,95

Lab Annealed (1,800 F.)

lVlill Annealed (1,950

Lab Annealed (1,800

Mill Annealed (1,950

Lab Annealed (1,800 F).

Samples of each were immersed in aqueous potassium hydroxide solutions. The potassium hydroxide baths employed were in concentrations ranging from 25% to 90% KOH and additions of from 6% to 10% potassium nitrate were made when TiS Al-2V2 Sn alloy was treated. Bath temperatures were varied from 300 F. to 450 F. Samples were immersed for a period of from 5 to 20 minutes. After each exposure all samples were given a 20% nitric-2% hydrofluoric acid, balance water, dip at 130 F. to remove loose adhering scale and to brighten metal surfaces. All bath concentrations investigated resulted in either partial or complete removal of oxide films from the samples tested; however, the solutions containing from 60% to 90% KOH removed the scale from the most heavily scaled sarnples, whereas incomplete descaling was shown in a number of cases when solutions containing less than 60% KOH were employed. The most efiective scale removal was obtained employing bath concentrations containing KOH at a temperature of 400 F. The tightest of scales such as are formed on muflie annealed Ti2% Fe2% Cr-2% Mo, hot rolled Ti5% Al-2V2 Sn and annealed Type 350, 355, 442 and 446 stainless steels were completely removed with exposures of ten minutes. While the scale was successfully removed employing the KOH baths containing from 60% to KOH no visible chemical or electrochemical corrosion or attack of the metal surfaces was evident.

Treatments to remove grind lines were equally success- Ground Ti4% Al-3% ful as those described above. Mo-1% V and Ti6% Al4% V alloys treated for ten minutes in aqueous 80% KOH at 400 F., satisfactorily passed 3XT bends, as ground materials generally fall a 30, 3XT bend.

Microscopic examination revealed no surface defects on the materials treated in aqueous KOH for either scale or grind line removal. The conditions of TiA(Ti 2% Fe2% Cr2% Mo) grooving and Ti-4% Al 3% Mo--'l% V pitting, commonly encountered in Virgo salts, were definitely not in evidence nor was there any indication of etching on specimens of chrome-manganese steels.

Production metal sheet products were descaled in a gallon stainless steel tank that contained 2,800 pounds of'a commercial grade of caustic potash (potassium hydroxide) plus water. The KOH concentration was from 70% to 75% (by weight); the temperature was maintained at from 400 F. to 425 -F. Results were as follows:

Production and Experimen tal ResultsDescaling and Pickling in Aqueous KOH [KOH concentration 70% to 75% (by wt); temperature 400 F. to 425 F.]

KOH No. Immer- Alloy Condition Size Sheets sion Acid Dip Results Time, minutes Mluglg AnF )nealed (1,300 F. Production Sheet 22 5 {5 mins. gfigiiflfi-f fl- P fi pl 130816 v 3mins. l a emova T1-75A Hot Rolled do 36 7 {5 mins. gggn fl..- Do

3 mins. 1 s T15% A12%% Sn Brit figu ed (AOF) (1,900 F. 1 it. x 3 it 2 10 mine. g egiu um D mine. a Ti-% Al2%% Sn HtlltgsjgllFecg (AOR) (1,900" F. 1 it. X 2% it 2 20 mins. gigginfii fln- Plugomplete Scale u nuns. aemova I-6% A1-4% V Hg? Rolled (l,700 F.-1,750 1 it. X 3 it l mine. g84 ---[-F- }G%np1 te 1 0318 mms. emova Til40A As Ground (150 Grit) 1 it. x 1 ft 1 1O Acid Scrub Bendaoility Imgrovettil to Pass peci ca ions. T 5% Al2%% Sn o int. x 1 it 2 10 do Do. T 6% Al4% V AS Ground (120 Grit) 1 it. X 1 it"- 2 5 D0. Ti2% Al6% M0-2% V 0 6 ft. X 6 it l D0.

It is to be understood that the process of this invention contemplates the employment of an aqueous solution of potassium hydroxide. Despite the chemical similarity and equivalency in many other applications of potassium hydroxide and sodium hydroxide, these salts are not equivalents in the process of this invention and sodium hydroxide will not produce the effective descaling characterized by solutions containing 60% to 90% KOH. The precise reason for the superiority of KOH over NaOH is not completely understood; however, the following table shows the results obtained when comparable solutions of 3 KOH and NaOH were used as descaling baths for various titanium alloys having heavy and refractory scales on their surfaces. The materials descaled were obtained from a commercial titanium fabricating plant in the hot rolled and roughed down conditions. Scales were intact. The cycle used to investigate the descaling ability of the respective baths included a 10 minute bath immersion, a water rinse, a 5 minute treatment in H 50 (20%, by volume, C.-P. acid) at 150 F., and a 3 minute treatment in 10% HNO 1.5% HF (by volume, C.P. acids) at 110 F. Two such complete cycles were given to specimens treated with the bath containing 80% NaOH and 10% NaNO in an efiort to completely remove the scale. In all other cases, only a single cycle treatment was used. After each descaling operation the respective descaling properties of baths containing sodium or potassium hydroxide were determined by making direct comparisons of the specimens treated. In the tabulation below the symbol C indicates complete descaling and the symbol X indicates incomplete descaling.

Alloy Treated pending application Serial No. 699,886, filed December 2, 1957, now abandoned.

We claim:

1. In a process for treating a metal article having a metal oxide scale on its surface to loosen and remove the 0 scale, the improvement which comprises; bringing said article into contact with an aqueous bath that contains from 60% to 90% potassium hydroxide and at least 10% water while maintaining said bath at a temperature of from 300 F. to 450 F.

5 2. In a process for treating a metal article having a metal oxide scale on its surface to loosen and remove the scale, the improvement which comprises; bringing said article into contact with an aqueous bath that contains from 60% to 90% potassium hydroxide, a nitrate content in an amount; equivalent to from 6% to 30% potassium nitrate, and at least 10% water, While maintaining said bath at a temperature of from 300 F. to 450 F.

3. In a process for treating a metal article having a metal oxide scale on its surface to loosen and remove the scale, the improvement which comprises; bringing said article into contact with an aqueous bath that contains from to 90% potassium hydroxide, from 6% to 30% potassium nitrate, and at least 10% water, while maintaining said bath at a temperature of from 300 F. to 450 F.

4. In a process for treating a metal article having a metal oxide scale on its surface formed from a metal of the class titanium, titanium alloys, stainless steel and 5 superalloys to loosen and remove the scale, the improvement which comprises; bringing said article into contact Commercially Ti47, Al- Ti5% Al Ti13% V Pure Ti 3% Mo-l% V 2%7 Sn 11% (Jr-3% Al Bath Hot Rough- Hot Rough- Hot Rongh- Hot Rough- Rolled down Rolled down Rolled down Rolled down NaOH+20% H O 425 F X G O O X X X X 80% KNO(I)|I20%;1g%I@O%E53; fi Riau- C C C O C C O C 80 "a 10 1 a. a

$55 .l X o o X X X X X 80% KOH+10% H2O+10% KNOa 425 F O C O O C C C C As is obvious from the above, complete descaling of all specimens was experienced only with the baths containing 80% potassium hydroxide. It is important to note that the sodium hydroxide solution of equivalent concentra tion was ineffective to remove scale from a number of the with an aqueous bath that contains from 60% to potassium hydroxide and at least 10% water while maintaining said bath at a temperature of from 300 F. to 450 F.

5. In a process for treating a metal article having a metal oxide scale on its surface formed from a metal of the class. titanium, titanium alloys, stainless steel and superalloys to loosen and remove the scale, the improvement which comprises; immersing said article in an aqueous bath that contains from 60% to 90% potassium hydroxide and at least water for a period of from 5 to minutes While maintaining said bath at a temperature of from 300 F. to 450 F.

6. 'In a process for treating a metal article having a metal oxide scale on its surface formed from a metal of the class titanium, 'tanium alloys, stainless steel and superalloys to loosen and remove the scale, the improvement Which comprises; bringing said article into contact with an aqueous bath that contains from 60% to 90% potassium hydroxide, a nitrate content in an amount equivalent to from 6% to potassium nitrate and at least 10% Water, While maintaining said bath at a temperature of from 300 F. to 450 F.

7. In a process for treating a metal arti le having a metal oxide scale on its surface formed from a metal of the class titanium, titanium alloys, stainless steel and superalloys to loosen and remove the scale, the improvement which comprises; immersing said article in an aqueous bath that contains from to'9.0 potassium hydroxide, a nitrate content in an amount equivalent to from 6% to 30% potassium nitnate'and at least 10% water for a period of from 5 to 20 minutes While maintaining said bath at a temperature of from 300 F. to 450 F.

8. In a process for removing grind lines from metals and alloys, the improvement which comprises; immersing said metals and alloys in an aqueous bath that consists essentially of from 60% to potassium hydroxide and at least 10% of Water while maintaining said bath at a temperature of from 300 F. to 450 F.

Re ren Cit in e fi e of thi p t nt UNITED STATES PATENTS UNITED STATES PATENT OFFICE Certificate Patent No. 3,121,026 Patented February 11, 1964: J aok M. Beigay, Roy C. Bongartz and Donald R. Zaremski Application having been made jointly by Jack M. Beigay, Roy G. Bongartz and Donald R. Zaremski, the inventors named in the patent above identified; and Allegheny Ludlum Steel Corporation, Brackenridge, Pennsylvania, a corporation of Pennsylvania the assignee, for the issuance of a certificate under the provisions of Title 35, Section 256 of the United States Code, deleting the names of the said Jack M. Beigay and Roy G. Bongartz from the patent as joint inventors, and a showing and proof of facts satisfying the requirements of the said section having been submitted, it is this 26th day of October 1965, certified that the names of the said Jack M. Beigay and Roy C. Bongartz are hereby deleted from the said patent as joint inventors with the said Donald R. Zaremski.

[SEAL] EDWIN L. REYNOLDS, First Assistant C'ommz'ssioner 0 f Patents. 

1. IN A PROCESS FOR TREATING A METAL ARTICLE HAVING A METAL OXIDE SCALE ON ITS SURFACE TO LOOSEN AND REMOVE THE SCALE, THE IMPROVEMENT WHICH COMPRISES; BRINGING SAID ARTICLE INTO CONTACT WITH AN AQUEOUS BATH THAT CONTAINS FROM 60% TO 90% POTASSIUM HYDROXIDE AND AT LEAST 10% WATER WHILE MAINTAINING SAID BATH AT A TEMPERATURE OF FROM 300*F. TO 450*F. 